The engines might last 500,000 miles miles but the bits bolted on don't. I used to get cars from fleet auctions for the taxi trade and they averaged 100,000k with a full mdsh. Within the 2 years it would be water pumps, alternators and half of them the dreaded fuel pump. Last one had a timing belt snap after on 13000 miles. That was a new cylinder head.
So for me it's buy new. Get in and start, no worry's. 5-6 years later weigh in and buy new again. Tax allowances are there too.
Same with cars now. Buy new and keep for 10 years. Even then only 60k on the clock.
But thats me. A decent well looked after private owned car maybe could be a good buy but a van makes me a living a car cost me my living.